The result of radiographic methods such as, for example, computed tomography, mammography, angiography, the X-ray inspection technique or comparable methods, is firstly the representation of the attenuation of an X-ray beam along its path from the X-ray source to the X-ray detector in a projection image. This attenuation is caused by the trans-irradiated materials along the beam path. Thus, the attenuation can also be understood as a line integral over the attenuation coefficients of all the voxels along the beam path.
Particularly in the case of tomography methods, for example in X-ray computed tomography (CT), it is possible to employ reconstruction methods to calculate backwards from the projected attenuation data to the attenuation coefficients μ of the individual voxels, and thus to attain a substantially more sensitive examination than in the case of simply viewing projection images.
Instead of the attenuation coefficient μ, in order to represent the attenuation distribution use is generally made of a value, the so-called CT number normalized to the attenuation coefficient of water. This CT number is calculated from an attenuation coefficient μ, currently determined by measurement, and the reference attenuation coefficient μH2O using the following equation:
      C    =          1000      ×                                    μ            -                          μ              H2O                                            μ            H2O                          ⁡                  [          HU          ]                      ,Where the CT number C is in Hounsfield units [HU]. The result for water is a value of CH2O=0 HU, and for air a value of CL=−1000 HU. Since both representations can be transformed into one another or are equivalent, in what follows the generally selected term of attenuation value or attenuation value coefficient denotes both the attenuation coefficient μ and the CT value.
Modern tomography machines such as, for example, X-ray computed tomography machines or C arc machines are used for recording and evaluating images in order to represent the three-dimensional attenuation distribution. X-ray computed tomography machines generally have a recording system with an X-ray tube and a detector, situated opposite the latter, for detecting the radiation emanating from the X-ray tube and penetrating the object. The recording system rotates several times about the examination object during recording.
C arc machines, which are frequently used for imaging during surgical operations, include one or two so-called C arc systems as recording systems that are each moved through an angle >180° about the object to be examined during recording of the image data. The measured data supplied by the recording systems are further processed in an evaluation unit in order to obtain the desired tomogram or volumetric image of the examination area.
U.S. Pat. No. 4,991,190 A also discloses an X-ray computed tomography machine that has a number of recording systems capable of revolving about a common rotation axis. The advantage of such tomography machines having a number of recording systems by comparison with a machine with only one recording system resides in the increased data recording rate, which leads to a shorter recording time and/or increased temporal resolution. A shortened recording time is advantageous because this minimizes movement artifacts in the reconstructed image, these artifacts possibly being caused by movement of the patient or of the patient's organs such as, the heart, for example, while image data are being recorded. An increased temporal resolution is required, for example, in order to represent movement cycles when the data required for reconstructing an image need to be recorded in the shortest possible time. An imaging tomography unit having at least two recording systems is also disclosed, for example, from DE 103 03 565, which is not a prior publication.
The attenuation value distribution of such X-ray images cannot, however, be used to deduce the material composition of an examination object, since the X-ray absorption is determined both by the effective atomic number of the material and by the material density. Materials and/or tissues of different chemical and physical composition can therefore exhibit identical attenuation values in X-ray images.
In order to enhance the informativeness of an X-ray image based on the local attenuation coefficient, it is therefore known, for example from U.S. Pat. No. 4,247,774 A, to use mutually differing X-ray spectra or X-ray quantum energies to produce an X-ray image. This method used in the field of computed tomography and generally also denoted as 2-spectra CT utilizes the fact that materials of higher atomic number absorb low-energy X-radiation much more strongly than materials of lower atomic number. By contrast, in the case of higher X-ray energies the attenuation values are equal to one another and are predominantly a function of the material density. By calculating the differences in the X-ray images recorded in conjunction with different X-ray tube voltages, it is therefore possible to obtain additional information relating to the materials on which the individual image areas are based.
Yet more specific items of information are obtained when, in addition, the method of so-called base material decomposition is applied to X-ray images. In this method the X-ray attenuation values of an object to be examined are measured with the aid of X-ray beams of lower and higher energy, and the values obtained are compared with the corresponding reference values of two base materials such as, for example, calcium for skeletal material and water for soft part tissues. It is assumed here that each measured value can be represented as a linear superposition of the measured values of the two base materials. Thus, a skeletal component and a soft tissue component can be calculated for each element of the pictorial representations of the object to be examined from the comparison with the values of the base materials, thus enabling a transformation of the original pictures into representations of the two base materials.
German patent application DE 101 43 131 A1 further discloses a method whose sensitivity and informativeness further exceeds the base material decomposition and, for example, enables a functional CT imaging of high informativeness. The method can be used to calculate the spatial distribution of the density ρ(r) and the effective atomic number Z(r) by evaluating spectrally influenced measured data of an X-ray apparatus, also denoted as ρ-Z projection below. Body constituents such as, for example, iodine or the like can be determined quantitatively from a combined evaluation of the distribution of the density and effective atomic number and, for example, instances of calcification can be removed by segmentation on the basis of the atomic number.
The recording of the image data with different spectral distributions that is necessary in the case of the last-named methods is frequently implemented by operating the X-ray source of the recording system successively with different tube voltages. It is also known to use different radiation filters or energy-sensitive detectors. However, these techniques favor the disturbing influence of patient movement, require a longer scanning time and also an increased administration of contrast medium in the case of CT examinations based on contrast medium.